Conventional such overvoltage protectors usually comprise a pair of gas tubes mounted coaxially within a housing. Fusible elements, typically discs of solder, are associated one with each of the gas tubes. The arrangement is such that, when an overload condition persists, for example when a power line contacts the telephone line, the heat generated in the gas tube will cause the fusible element to melt and short-circuit the gas tube, either directly or by releasing a spring-loaded plunger.
In U.S. Pat. No. 4,056,840, issued November 1977, P. S. Lundsgaard et al disclose such a protector having coaxial gas tubes with a fusible dielectric pellet mounted directly upon each gas tube. Melting of the dielectric pellet allows a resilient conductive member to short-circuit the gas tube.
In U.S. Pat. No. 4,851,957, issued July 1989, K. H. Chung discloses a lead or plastics pellet mounted directly upon a gas tube. The pellet maintains a contact member away from the electrodes of the gas tube. When the pellet melts, the contact member short-circuits the gas tube.
An alternative overvoltage protector is disclosed in U.S. Pat. No. 4,212,047 by Napiorkowski, issued Jul. 8, 1980, to which the reader is directed for reference. Napiorkowski discloses a protector having a sleeve of fluoroplastics material around the gas tube and a clip of spring metal surrounding the sleeve. When a sustained fault occurs, the heat generated causes the fluoroplastics material to melt, allowing the metal clip to contact the gas tube and effect the desired short circuit.
In practice, these known devices are susceptible to problems concerning the suitability of plastics material for use in overvoltage protectors of the kind used in telecommunications, especially in central offices and at subscriber's premises. As dicussed in U.S. Pat. No. 4,056,840, such protectors are designed to be "self-restoring" i. e. return to its open-circuit condition once the fault has been cleared. Typically, such a protector can be expected to operate many times during a useful life of as long as forty years without being subjected to a fault severe enough and sustained long enough, to fuse the plastics material and short-circuit the gas tube. Although such repeated operations do not generate enough heat to melt heat the plastics material, the plastics material nevertheless is subjected repeatedly to relatively high temperatures because it is in direct contact with the gas tube. This can lead to creepage of the plastics material and to premature failure.
The plastics material should also preferably exhibit a clearly defined and abrupt transition between its solid and molten states. The range of plastics materials which exhibit these characteristics and are capable of withstanding the relatively high temperatures associated with direct contact with the gas tube or other protection device is limited.